Many Biopharmaceutics Classification System (BCS) class III or IV drugs suffer from the lack of gastrointestinal (GI) tract membrane permeability leading to poor oral bioavailability. Different strategies have been implemented to improve the permeability and subsequently the oral bioavailability of such drugs. For example, the U.S. patent application 20060068010 describes a formulation method for improving the permeability of drugs and subsequently increasing their bioavailability by granulation of the physical solid mixture of the drug with one or more amino acids, at least one inter-granular hydrophilic polymer, and an additional immediate release excipient. Another application WO 200602009 A1 disclosed an increase in the oral bioavailability of poorly permeable bisphosphonate drugs; risedronate, an exemplary bisphosphonate, was mixed with a chelating agent such as ethylenediaminetetraacetic acid (EDTA) and other excipients to make an oral dosage form with enhanced bioavailability. In another application, WO 2007093226 describes a method for improving the bioavailability of ibandronate by generating a physical mixture of the drug together with a modified amino acid (acylation or sulphonation of the amino group with phenyl or cyclohexyl) and other excipients. Another application, WO 2003007916 A1, reports a gastric retention system to improve the bioavailability of a poorly permeable drug, alendronate, which was orally formulated with vitamin D and released an hour after the immediate release of vitamin D. WO 2006080780 discloses yet another method to improve the permeability and bioavailability of alendronate by mixing it with a biocompatible cationic polymer (i.e. water soluble chitosan) with up to a 10:1 weight ratio of the chitosan to the drug, while the resulting mixture can be formulated into a solid or liquid oral dosage form. An additional method of improving permeability of drug materials was discussed in the U.S. patent application 2007/014319 A1, where an oral dosage form was formulated by a powder mixture of a bisphosphonic acid (e.g. zoledronic acid) together with an inactive ingredient (either an ester of a medium chain fatty acid or a lipophilic polyethylene glycol ester). A similar approach was disclosed in the US application 2007/0238707 A1 where a medium chain length fatty acid or its derivative (6-20 carbon atom fatty acid chain) was physically mixed with a poorly permeable drug (e.g. zoledronic acid) in a capsule that was enterically coated.
Zoledronic acid, known as (1-hydroxy-2-imidazol-1-yl-1-phosphono-ethyl)phosphonic acid, is depicted by the following chemical structure:

Zoledronic acid is a third generation bisphosphonate which far exceeds the previous generations in terms of efficacy and is used predominately for indications of osteoporosis, Paget's disease, hypercalcemia, and inhibition of bone metastasis. It was originally developed by Novartis and marketed as the monohydrate under the brand names Zometa® and Reclast®. Zoledronic acid was first approved in 2000 for the treatment of hypercalcemia in Canada. It was later approved for use in the US for hypercalcemia in 2001, for multiple myeloma and bone metastases from solid tumors in 2002, and for osteoporosis and Paget's disease in 2007. Clinical trials have also been conducted and are on-going to explore the use of zoledronic acid in neoadjuvant or adjuvant cancer therapy, Coleman, et al., British J Cancer 2010; 102(7):1099-1105, Gnant, et al., New England J Medicine. 2009, 360 (17):679-691 and Davies, et al. J Clinical Oncology, 2010, 28(7s): Abstract 8021. Zoledronic acid is administered as an intravenous (IV) dose of 4 mg over 15 minutes for hypercalcemia of malignancy, multiple myeloma, and bone metastases from solid tumors, while an IV dose of 5 mg over 15 minutes is used for osteoporosis and Paget's disease.
Zoledronic acid is sparingly soluble in water and 0.1 N HCl solution but is freely soluble in 0.1 N NaOH. Zoledronic acid is practically insoluble in various organic solvents. Much effort has been taken to generate novel oral formulations of zoledronic acid through crystallization and metal salt formation to improve its aqueous solubility, permeability, and subsequent oral bioavailability. A crystalline trihydrate was disclosed in the U.S. Patent application 2006/0178439 A1 and world patent application WO2007/032808. Seven hydrated forms, an amorphous form, three monosodium salts, and eleven disodium salts with varying degrees of hydration of zoledronic acid were also disclosed in the patent application WO2005/005447 A2. Zoledronate metal salts including Na+, Mg2+, Zn2+ were reported in the journal of Drugs of the Future (Sorbera et al, Drugs of the Future, 2000, 25(3): 259-268). Zoledronate, zoledronic, or zoledronic salt represents the ionic form of zoledronic acid. Patent application WO2008/064849 A1 from Novartis disclosed additional metal salts including two Ca2+ salts, two Zn2+ salts, one Mg2+ salt, as well as a monohydrate, a trihydrate, an amorphous form, and an anhydrous form.
According to the US Food and Drug Administration (FDA) Summary Basis of Approval (SBA) for zoledronic acid, the poor oral bioavailability (approximately 1%), is partially due to its poor permeability in the GI tract. It was also noted that insoluble metal complexes were formed in the upper intestines, most commonly with calcium. Zoledronic acid has also been shown to cause severe gastric and intestinal irritations.
All of the above attempts to improve the oral bioavailability of zoledronic acid were either focused on improving the aqueous solubility by generating novel solid forms, or by mixing the drug with an inactive ingredient that has enhanced GI tract permeability. The improvement of aqueous solubility failed to improve the bioavailability of zoledronic acid, since the formation of insoluble zoledronate calcium complexes is unlikely to be prevented. On the other hand, powder mixtures of the poorly permeable drug with inactive permeability enhancers improved the bioavailability of the drug. This approach of mixing different materials with different particle sizes and size distributions could result in poor blend/physical mixture uniformity. Constituents of the mixture could also segregate during transportation or with shaking and vibration. Additionally, the powder blends require rigorous batch-to-batch consistency to ensure the uniformity of the blend batches.
The upward trend in the use of oral drugs continues especially in light of the goal to decrease the overall cost of healthcare. Orally administered drugs are becoming more preferred in various therapeutic areas including oncology. Clearly, there is an opportunity to create oral dosage forms of drugs with poor aqueous solubility and/or poor permeability. One such example is zoledronic acid which is only approved for intravenous administration due to its low oral bioavailability, resulting from poor permeability. By using pharmaceutically acceptable and/or approved coformers to hydrogen or ionically bond with an API, novel molecular complexes (e.g. cocrystals, salts, solvates, and mixtures thereof) with improved solubility and/or permeability can be created. These novel molecular complexes could be used in the development of novel oral dosage forms of BCS Class III and IV drugs.